Dannager
First Post
This, I think, is the crux of the issue, though.Making up your own plots is all well and good, but DMs can make their own monster stat blocks, too, and yet we've got more than 300 of them (more than most people will use in a year of D&D).
Tons of DMs are great at making up their own plots. All that requires is a halfway-decent imagination, which most DMs have.
Tons of DMs suck at making up their own monster stat blocks. That requires deep rules knowledge, in-depth play experience, a mind for math, and the ability to either playtest your creation or anticipate its performance in a tactical encounter.
Given this disparity, it makes sense to give DMs assistance where they need it most - in coming up with compelling, effective mechanics for monsters.